Method and apparatus for monitoring beam profile and power

ABSTRACT

A system and a method for monitoring a beam in an inspection system are provided. The system includes an image sensor configured to collect a sequence of images of a beam spot of a beam formed on a surface, each image of the sequence of images having been collected at a different exposure time of the image sensor, and a controller configured to combine the sequence of images to obtain a beam profile of the beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. application 62/730,972 whichwas filed on Sep. 13, 2018, and which is incorporated herein in itsentirety by reference.

TECHNICAL FIELD

The present disclosure generally relates to the field of chargedparticle beam systems and, more particularly, to a method and anapparatus for monitoring beam profile and beam power of a laser beamused in a charged particle beam system.

BACKGROUND

In manufacturing processes of integrated circuits (ICs), unfinished orfinished circuit components are inspected to ensure that they aremanufactured according to design and are free of defects. An inspectionsystem utilizing an optical microscope typically has resolution down toa few hundred nanometers; and the resolution is limited by thewavelength of light. As the physical sizes of IC components continue toreduce down to sub-100 or even sub-10 nanometers, inspection systemscapable of higher resolution than those utilizing optical microscopesare needed.

A charged particle (e.g., electron) beam microscope, such as a scanningelectron microscope (SEM) or a transmission electron microscope (TEM),capable of resolution down to less than a nanometer, serves as apracticable tool for inspecting IC components having a feature size thatis sub-100 nanometers. With a SEM, electrons of a single primaryelectron beam, or electrons of a plurality of primary electron beams,can be focused at locations of interest of a wafer under inspection. Theprimary electrons interact with the wafer and may be backscattered ormay cause the wafer to emit secondary electrons. The intensity of theelectron beams comprising the backscattered electrons and the secondaryelectrons may vary based on the properties of the internal and externalstructures of the wafer, and thereby may indicate whether the wafer hasdefects.

SUMMARY

Embodiments consistent with the present disclosure include systems,methods, and non-transitory computer-readable mediums for monitoring abeam in an inspection system. The system includes an image sensorcollect a sequence of images of a beam spot of a beam formed on asurface. Each image of the sequence of images has been collected at adifferent exposure time. of the image sensor. The system also includes acontroller configured to combine the sequence of images to obtain a beamprofile of the beam.

Embodiments consistent with the present disclosure include systems,methods, and non-transitory computer-readable mediums for monitoring abeam in an inspection system. The system includes an image sensorconfigured to collect an image of a beam spot of a beam formed on asurface, and a controller configured to transform coordinates of theimage based on positions of the beam and the image sensor with respectto the beam spot and a magnification factor of an optical systemarranged between the image sensor and the surface.

Embodiments consistent with the present disclosure include systems,methods, and non-transitory computer-readable mediums for monitoring abeam in an inspection system. The system includes an image sensorconfigured to collect a plurality of images of a beam spot of a beamformed at different locations on a surface, and a controller configuredto generate an averaged image based on the plurality of images.

Embodiments consistent with the present disclosure include systems,methods, and non-transitory computer-readable mediums for monitoring abeam in an inspection system. The system includes an image sensorconfigured to collect an image of a beam spot of a beam formed on asurface, and a controller configured to obtain a beam profile of thebeam based on the image, obtain a total grey level of the beam spotbased on the beam profile, and determine a power of the beam based on apredetermined relationship between the total grey level and the power.

Additional objects and advantages of the disclosed embodiments will beset forth in part in the following description, and in part will beapparent from the description, or may be learned by practice of theembodiments. The objects and advantages of the disclosed embodiments maybe realized and attained by the elements and combinations set forth inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosed embodiments, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary electron beam inspection (EBI) system100, consistent with embodiments of the present disclosure.

FIG. 2A and FIG. 2B are schematic diagrams illustrating exemplaryelectron beam tools, consistent with embodiments of the presentdisclosure that may be a part of the exemplary electron beam inspectionsystem of FIG. 1

FIG. 3A is a side view of an inspection system, consistent withembodiments of the present disclosure.

FIG. 3B is a top view of the inspection system of FIG. 3A, consistentwith embodiments of the present disclosure.

FIG. 4 is an exemplary flowchart of a process for transforming an imageof a beam spot by using a coordinate transformation method, consistentwith embodiments of the present disclosure.

FIG. 5A is an exemplary image of a beam spot captured by an imagesensor, consistent with embodiments of the present disclosure.

FIG. 5B is an exemplary image of a Top View of a beam spot formed on asample surface, consistent with embodiments of the present disclosure.

FIG. 6 is diagram of a 3D beam profile obtained based on an image of abeam spot formed on a relatively rough sample surface, consistent withembodiments of the present disclosure.

FIG. 7 is an exemplary flowchart of a process for obtaining an averagedbeam spot image, consistent with embodiments of the disclosure.

FIG. 8A is an example of different locations where beam spot images canbe captured, consistent with embodiments of the present disclosure.

FIG. 8B is a diagram of a 3D beam profile obtained based on an averagedbeam spot image, consistent with embodiments of the disclosure.

FIG. 9 is an exemplary flowchart of a process for obtaining a beamprofile using a dynamic range extension method, consistent withembodiments of the present disclosure.

FIGS. 10A-10C are examples of beam spot images taken at differentexposure times, consistent with embodiments of the present disclosure.

FIGS. 10D-10F are examples of partial beam profiles obtained from thebeam spot images of FIGS. 10A-10C, consistent with embodiments of thepresent disclosure.

FIG. 10G is a complete beam profile obtained based on the beam spotimages FIGS. 10A-10C, consistent with embodiments of the presentdisclosure.

FIGS. 11A-11G illustrate an exemplary method of determining a grey levelmagnification factor, consistent with embodiments of the presentdisclosure.

FIG. 12 is an exemplary flowchart of a process for determining a beampower using a power calibration method, consistent with embodiments ofthe present disclosure.

FIG. 13A is an example of a 3D beam profile, consistent with embodimentsof the present disclosure.

FIG. 13B schematically illustrates a method of measuring a beam power bya power meter, consistent with some embodiments of the presentdisclosure.

FIG. 13C is a diagram of an exemplary relationship between beam powerand total grey level of a beam spot, consistent with some embodiments ofthe present disclosure.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings. The followingdescription refers to the accompanying drawings in which the samenumbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in thefollowing description of exemplary embodiments do not represent allimplementations consistent with the invention. Instead, they are merelyexamples of apparatuses and methods consistent with aspects related tothe invention as recited in the appended claims.

The enhanced computing power of electronic devices, while reducing thephysical size of the devices, can be accomplished by significantlyincreasing the packing density of circuit components such as,transistors, capacitors, diodes, etc. on an IC chip. For example, in asmart phone, an IC chip (which is the size of a thumbnail) may includeover 2 billion transistors, the size of each transistor being less than1/1000th of a human hair. Not surprisingly, semiconductor ICmanufacturing is a complex process, with hundreds of individual steps.Errors in even one step have the potential to dramatically affect thefunctioning of the final product. Even one “killer defect” can causedevice failure. The goal of the manufacturing process is to improve theoverall yield of the process. For example, for a 50-step process to get75% yield, each individual step must have a yield greater than 99.4%,and if the individual step yield is 95%, the overall process yield dropsto 7%.

Defects may be generated during various stages of semiconductorprocessing. For the reason stated above, it is important to find defectsaccurately and efficiently as early as possible. A charged particle(e.g., electron) beam microscope, such as a scanning electron microscope(SEM), is a useful tool for inspecting semiconductor wafer surfaces todetect defects. During operation, the charged particle beam microscopescans a primary charged-particle beam, such as an electron beam(e-beam), over a semiconductor wafer held on a stage, and generates animage of the wafer surface by detecting a secondary charged-particlebeam reflected from the wafer surface. When the charged-particle beamscans the wafer, charges may be accumulated on the wafer due to largebeam current, which may negatively affect the quality of the image. Toregulate the accumulated charges on the wafer, an Advanced ChargeController (ACC) module is employed to illuminate a light beam, such asa laser beam, on the wafer, so as to control the accumulated charges dueto photoconductivity and/or photoelectric effect. It is thus importantto monitor the power and quality of the light beam, so as to effectivelycontrol the accumulated charges.

Conventionally, the power and quality of a light beam are monitored by apower meter and a beam profiler. However, during operation of a chargedparticle beam microscope, the microscope, the ACC module thatilluminates the light beam, and the stage that holds the semiconductorwafer are disposed in a vacuum chamber. Due to the limited space insidethe vacuum chamber, the power meter and the beam profiler cannot bedisposed in the vacuum chamber. As a result, the power meter and thebeam profiler cannot be used to monitor power and quality of the lightbeam during the operation of a charged particle beam microscope.

To monitor the light beam in a limited space in a vacuum chamber, thedisclosed system uses an image sensor, which is already included in thecharged particle beam microscope for other purposes (e.g., observingwafer surface, etc.), to monitor the power and profile of the light beamemitted from the ACC module.

Due to the high intensity of the light beam (e.g., laser beam), an imagesensor may not have a sufficiently large dynamic range to capture thecomplete beam profile of the light beam. In order to solve this problem,the disclosed systems can configure the image sensor to collect duringdifferent exposure times a sequence of images of a beam spot formed onthe semiconductor wafer, resulting in a set of images capturing thecomplete beam profile. As a result, even if the image sensor has adynamic range that is not sufficiently large to capture the completebeam profile of the light beam, the controller can still obtain thecomplete beam profile based on the partial beam files obtained from thesequence of images.

Due to limited space inside the vacuum chamber of the microscope, theimage sensor may not be able to collect images from a top side of asemiconductor wafer. Thus, the shape of the beam spot in the image takenby the image sensor may not be a real beam projection on the wafer. Tosolve this problem, the disclosed systems can transform coordinates ofan image based on positions of the light and the image sensor withrespect to the beam spot and a magnification factor of an optical systemarranged between the image sensor and the wafer surface. As a result, aprojection of the light beam on the surface may be obtained, and aninteraction between the light beam and the surface can be accuratelyevaluated.

In some instances, the ACC module, the beam spot, and the image sensormay not be in the same plane. Therefore, the light beam cannot bedirectly reflected into the image sensor. In order to solve thisproblem, the disclosed system can emit the light beam onto a relativelyrough surface, such as a corner of a wafer stage, and the image sensormay be configured to collect light scattered or diffracted from thisrough surface. In this case, due to the rough surface, a beam profileobtained from the captured image is also rough. In order to solve thisproblem, the disclosed systems can configure the image sensor to capturea plurality of images of the beam spot formed at different locations onthe surface. Using these plurality of images, the disclosed systems cangenerate an averaged image based on the plurality of images, and use theaveraged image as the collected image for the at least one exposuretime. As a result, the beam profile obtained based on the averaged imagemay be smooth, even if the surface on which the beam spot is formed isrelatively rough.

In some instances, the disclosed system can emit the light beam onto arelatively smooth and mirror-like surface, and the light beam can bedirectly reflected into the image sensor. In this case, a single beamspot image captured by the image sensor can be used to obtain a beamprofile, and it is not necessary to generate an averaged image based ona plurality of images of the beam spot formed at different locations onthe surface.

Moreover, the disclosed systems can obtain a total grey level of thebeam spot, and determine a power of the light beam based on apredetermined relationship between the total grey level and the power.The disclosed systems can control a power of the ACC module based on thedetermined power of the light beam, in order to obtain a desired powerof the light beam.

FIG. 1 illustrates an exemplary electron beam inspection (EBI) system100 consistent with embodiments of the present disclosure. While thisand other examples refer to an electron beam system, it is appreciatedthat the techniques disclosed herein are applicable to systems otherthan electron beam systems, such as an ellipsometer, a velocimeter, aCO2 laser (e.g., for machining), non-electron beam systems where a laserneeds to be monitored but the space is limited, among others. As shownin FIG. 1, EBI system 100 includes a main chamber 101, a load/lockchamber 102, an electron beam tool 104, and an equipment front endmodule (EFEM) 106. Electron beam tool 104 is located within main chamber101. EFEM 106 includes a first loading port 106 a and a second loadingport 106 b. EFEM 106 may include additional loading port(s). Firstloading port 106 a and second loading port 106 b receive wafer frontopening unified pods (FOUPs) that contain wafers (e.g., semiconductorwafers or wafers made of other material(s)) or samples to be inspected(wafers and samples may be collectively referred to as “wafers” herein).

One or more robotic arms (not shown) in EFEM 106 may transport thewafers to load/lock chamber 102. Load/lock chamber 102 is connected to aload/lock vacuum pump system (not shown) which removes gas molecules inload/lock chamber 102 to reach a first pressure below the atmosphericpressure. After reaching the first pressure, one or more robotic arms(not shown) may transport the wafer from load/lock chamber 102 to mainchamber 101. Main chamber 101 is connected to a main chamber vacuum pumpsystem (not shown) which removes gas molecules in main chamber 101 toreach a second pressure below the first pressure. After reaching thesecond pressure, the wafer is subject to inspection by electron beamtool 104. Electron beam tool 104 may be a single-beam system or amulti-beam system. A controller 109 is electronically connected toelectron beam tool 104. Controller 109 may be a computer configured toexecute various controls of EBI system 100. While controller 109 isshown in FIG. 1 as being outside of the structure that includes mainchamber 101, load/lock chamber 102, and EFEM 106, it is appreciated thatcontroller 109 can part of the structure.

FIG. 2A illustrates a charged particle beam apparatus in which aninspection system may comprise a single primary beam that may beconfigured to generate a secondary beam. A detector may be placed alongan optical axis 105, as in the embodiment shown in FIG. 2A. In someembodiments, a detector may be arranged off axis.

As shown in FIG. 2A, an electron beam tool 104 may include a waferholder 136 supported by motorized stage 134 to hold a wafer 150 to beinspected. Electron beam tool 104 includes an electron emitter, whichmay comprise a cathode 103, an anode 120, and a gun aperture 122.Electron beam tool 104 further includes a beam limit aperture 125, acondenser lens 126, a column aperture 135, an objective lens assembly132, and an electron detector 144. Objective lens assembly 132, in someembodiments, may be a modified swing objective retarding immersion lens(SORIL), which includes a pole piece 132 a, a control electrode 132 b, adeflector 132 c, and an exciting coil 132 d. In an imaging process, anelectron beam 161 emanating from the tip of cathode 103 may beaccelerated by anode 120 voltage, pass through gun aperture 122, beamlimit aperture 125, condenser lens 126, and focused into a probe spot bythe modified SORIL lens and then impinge onto the surface of wafer 150.The probe spot may be scanned across the surface of wafer 150 by adeflector, such as deflector 132 c or other deflectors in the SORILlens. Secondary electrons emanated from the wafer surface may becollected by detector 144 to form an image of an area of interest onwafer 150.

There may also be provided an image processing system 199 that includesan image acquirer 200, a storage 130, and controller 109. Image acquirer200 may comprise one or more processors. For example, image acquirer 200may comprise a computer, server, mainframe host, terminals, personalcomputer, any kind of mobile computing devices, and the like, or acombination thereof. Image acquirer 200 may connect with detector 144 ofelectron beam tool 104 through a medium such as an electrical conductor,optical fiber cable, portable storage media, IR, Bluetooth, internet,wireless network, wireless radio, or a combination thereof. Imageacquirer 200 may receive a signal from detector 144 and may construct animage. Image acquirer 200 may thus acquire images of wafer 150. Imageacquirer 200 may also perform various post-processing functions, such asgenerating contours, superimposing indicators on an acquired image, andthe like. Image acquirer 200 may be configured to perform adjustments ofbrightness and contrast, etc. of acquired images. Storage 130 may be astorage medium such as a hard disk, random access memory (RAM), cloudstorage, other types of computer readable memory, and the like. Storage130 may be coupled with image acquirer 200 and may be used for savingscanned raw image data as original images, and post-processed images.Image acquirer 200 and storage 130 may be connected to controller 109.In some embodiments, image acquirer 200, storage 130, and controller 109may be integrated together as one control unit.

In some embodiments, image acquirer 200 may acquire one or more imagesof a sample based on an imaging signal received from detector 144. Animaging signal may correspond to a scanning operation for conductingcharged particle imaging. An acquired image may be a single imagecomprising a plurality of imaging areas that may contain variousfeatures of wafer 150. The single image may be stored in storage 130.Imaging may be performed on the basis of imaging frames.

The condenser and illumination optics of the electron beam tool maycomprise or be supplemented by electromagnetic quadrupole electronlenses. For example, as shown in FIG. 2A, the electron beam tool 104 maycomprise a first quadrupole lens 148 and a second quadrupole lens 158.In some embodiments, the quadrupole lenses are used for controlling theelectron beam. For example, first quadrupole lens 148 can be controlledto adjust the beam current and second quadrupole lens 158 can becontrolled to adjust the beam spot size and beam shape.

Although FIG. 2A shows electron beam tool 104 as a single-beaminspection tool that may use only one primary electron beam to scan onelocation of wafer 150 at a time, embodiments of the present disclosureare not so limited. For example, electron beam tool 104 may also be amulti-beam inspection tool that employs multiple primary electronbeamlets to simultaneously scan multiple locations on wafer 150.

For example, reference is now made to FIG. 2B, which illustrates anotherembodiment of electron beam tool 104. Electron beam tool 104 (alsoreferred to herein as apparatus 104) may comprise an electron source202, a gun aperture 204, a condenser lens 206, a primary electron beam210 emitted from electron source 202, a source conversion unit 212, aplurality of beamlets 214, 216, and 218 of primary electron beam 210, aprimary projection optical system 220, a wafer stage (not shown in FIG.2B), multiple secondary electron beams 236, 238, and 240, a secondaryoptical system 242, and an electron detection device 244. A controller,image processing system, and the like may be coupled to electrondetection device 244. Primary projection optical system 220 may comprisea beam separator 222, deflection scanning unit 226, and objective lens228. Electron detection device 244 may comprise detection sub-regions246, 248, and 250.

Electron source 202, gun aperture 204, condenser lens 206, sourceconversion unit 212, beam separator 222, deflection scanning unit 226,and objective lens 228 may be aligned with a primary optical axis 260 ofapparatus 104. Secondary optical system 242 and electron detectiondevice 244 may be aligned with a secondary optical axis 252 of apparatus104.

Electron source 202 may comprise a cathode, an extractor or an anode,wherein primary electrons can be emitted from the cathode and extractedor accelerated to form a primary electron beam 210 with a crossover(virtual or real) 208. Primary electron beam 210 can be visualized asbeing emitted from crossover 208. Gun aperture 204 may block offperipheral electrons of primary electron beam 210 to reduce size ofprobe spots 270, 272, and 274.

Source conversion unit 212 may comprise an array of image-formingelements (not shown in FIG. 2B) and an array of beam-limit apertures(not shown in FIG. 2B). An example of source conversion unit 212 may befound in U.S. Pat. No. 9,691,586; U.S. Publication No. 2017/0025243; andInternational Application No. PCT/EP2017/084429, all of which areincorporated by reference in their entireties. The array ofimage-forming elements may comprise an array of micro-deflectors ormicro-lenses. The array of image-forming elements may form a pluralityof parallel images (virtual or real) of crossover 208 with a pluralityof beamlets 214, 216, and 218 of primary electron beam 210. The array ofbeam-limit apertures may limit the plurality of beamlets 214, 216, and218.

Condenser lens 206 may focus primary electron beam 210. The electriccurrents of beamlets 214, 216, and 218 downstream of source conversionunit 212 may be varied by adjusting the focusing power of condenser lens206 or by changing the radial sizes of the corresponding beam-limitapertures within the array of beam-limit apertures. Condenser lens 206may be a moveable condenser lens that may be configured so that theposition of its first principle plane is movable. The movable condenserlens may be configured to be magnetic, which may result in off-axisbeamlets 216 and 218 landing on the beamlet-limit apertures withrotation angles. The rotation angles change with the focusing power andthe position of the first principal plane of the movable condenser lens.In some embodiments, the moveable condenser lens may be a moveableanti-rotation condenser lens, which involves an anti-rotation lens witha movable first principal plane. Moveable condenser lens is furtherdescribed in U.S. Publication No. 2017/0025241, which is incorporated byreference in its entirety.

Objective lens 228 may focus beamlets 214, 216, and 218 onto a wafer 230for inspection and may form a plurality of probe spots 270, 272, and 274on the surface of wafer 230.

Beam separator 222 may be a beam separator of Wien filter typegenerating an electrostatic dipole field and a magnetic dipole field. Insome embodiments, if they are applied, the force exerted byelectrostatic dipole field on an electron of beamlets 214, 216, and 218may be equal in magnitude and opposite in direction to the force exertedon the electron by magnetic dipole field when the electrons of the beamare traveling at a particular velocity. Beamlets 214, 216, and 218 cantherefore pass straight through beam separator 222 with zero deflectionangle. However, the total dispersion of beamlets 214, 216, and 218generated by beam separator 222 may also be non-zero such as when theelectrons of the beam are traveling at a velocity other than theparticular velocity. Beam separator 222 may separate secondary electronbeams 236, 238, and 240 from beamlets 214, 216, and 218 and directsecondary electron beams 236, 238, and 240 towards secondary opticalsystem 242.

Deflection scanning unit 226 may deflect beamlets 214, 216, and 218 toscan probe spots 270, 272, and 274 over a surface area of wafer 230. Inresponse to incidence of beamlets 214, 216, and 218 at probe spots 270,272, and 274, secondary electron beams 236, 238, and 240 may be emittedfrom wafer 230. Secondary electron beams 236, 238, and 240 may compriseelectrons with a distribution of energies including secondary electronsand backscattered electrons. Secondary optical system 242 may focussecondary electron beams 236, 238, and 240 onto detection sub-regions246, 248, and 250 of electron detection device 244. Detectionsub-regions 246, 248, and 250 may be configured to detect correspondingsecondary electron beams 236, 238, and 240 and generate correspondingsignals used to reconstruct an image of surface area of wafer 230.

FIG. 3A is a side view of an inspection system 300 consistent withembodiments of the present disclosure. As shown in FIG. 3A, inspectionsystem 300 includes an electron beam tool 310, an advanced chargecontroller (ACC) module 320, a stage 330 on which a sample to beinspected (e.g., a wafer 340) is disposed, and a beam monitoring system350. Electron beam tool 310 may emit a primary electron beam 312 onto anarea of interest on wafer 340, and collect secondary electrons emanatedfrom the wafer surface to form an image of the area of interest on wafer340. ACC module 320 may include an ACC emitter that emits a light beam322 (e.g., laser beam) onto wafer 340 and form a beam spot of light beam322 on the wafer surface. When primary electron beam 312 irradiates thearea of interest on wafer 340, charges may be accumulated due to a largebeam current. Light beam 322 emitted from ACC module 320 may beconfigured to regulate the accumulated charges due to photoconductivityor photoelectric effect, or a combination of photoconductivity andphotoelectric effect. It is important to monitor the power and qualityof light beam 322 emitted from ACC module 320 to effectively regulatethe accumulated charges by light beam 322.

Conventionally, the power and quality of a light beam are monitored by apower meter and a beam profiler. However, an inspection system 300 maynot be provided with a power meter or a beam profiler. As a result,power and quality of the light beam may not be monitored in situ, i.e.,during the operation of inspection system 300.

According to embodiments of the present disclosure, beam monitoringsystem 350 may be included in inspection system 300 to monitor the powerand profile of light beam 322 emitted from ACC module 320. Beammonitoring system 350 may include an image sensor 360, an optical system370 disposed between wafer 340 and image sensor 360, and a controller380. Optical system 370 may include one or more optical lenses that areconfigured to focus a secondary beam 324 onto image sensor 360.Secondary beam 324 may include light beam scattered from the wafersurface, or light beam diffracted from the wafer surface, or acombination of light beam scattered from the wafer surface and lightbeam diffracted from the wafer surface. Image sensor 360 may be aCharge-Coupled Device (CCD) camera or a ComplementaryMetal-Oxide-Semiconductor (CMOS) sensor that detects secondary beam 324to form an image of secondary beam 324. Controller 380 may be a computerconfigured to receive the image of secondary beam 324 from image sensor360 and, based on the image of secondary beam 324, obtain a beam profileand beam power of light beam 322 emitted from ACC module 320. Inaddition, controller 380 may be configured to control ACC module 320based on the beam profile and beam power of light beam 322. For example,based on the beam power of light beam 322, controller 380 mayautomatically adjust a working current of the ACC emitter included inACC module 320 to keep an output power of the ACC emitter at a targetpower or to remain stable. Meanwhile, based on the beam profile of lightbeam 322, controller 380 may adjust a beam shaper included in ACC module320 to obtain a desired shape of the beam profile or a desired powerdistribution. Moreover, based on the beam power of light beam 322,controller 380 may be configured to monitor a location of the beam spotformed by light beam 322 on the wafer surface. In case of misalignment,controller 380 may be configured to re-align the beam spot to a field ofview of electron beam tool 310. That is, controller 380 may control theACC emitter to emit light beam 322 to a position on wafer 340 that isirradiated by primary electron beam 312.

FIG. 3B is a top view of inspection system 300 during operation of beammonitoring system 350, consistent with embodiments of the presentdisclosure. To simplify the illustration, electron beam tool 310 andcontroller 380 are omitted from FIG. 3B. As shown in FIG. 3B, ininspection system 300, light beam 322 from ACC module 320 may be alignedwith the field of view of electron beam tool 310. Meanwhile, the fieldof view of image sensor 360 may also be aligned with the field of viewof electron beam tool 310. At the beginning of a beam monitoringprocess, the field of view of electron beam tool 310 may be moved to acorner portion of stage 330 on which wafer 340 is not disposed. Thecorner portion of stage 330 may have a relatively rough surface, andsecondary beam 324 may be scattered or diffracted from this roughsurface. In this way, image sensor 360 may capture images of the beamspot formed on the corner portion of stage 330. In some alternativeembodiments, a special designed sample surface may be used to collectthe light beam signal emitted from ACC module 320. Still alternatively,the wafer surface or a smooth surface may also be used to collect thelight beam signal emitted from ACC module 320.

In some embodiments, such as the embodiment illustrated in FIG. 3A, ACCmodule 320 and image sensor 360 are disposed outside of a vacuum chamber390, in which electron beam tool 310 and wafer 340 are disposed. Duringoperation of the inspection system 300, light beam 322 and secondarybeam 324 may pass through one or more windows formed in vacuum chamber390. In some alternative embodiments, at least one of ACC module 320 andimage sensor 360 may be disposed inside of vacuum chamber 390.

In some embodiments, such as the embodiments illustrated in FIGS. 3A and3B, image sensor 360 is not disposed directly on top of the beam spotformed by light beam 322 emitted from ACC module 320, and thus imagesensor 360 may not capture the image of the beam spot from a top side ofthe beam spot. As a result, the shape of the beam spot in the imagecaptured by image sensor 360 may not be the same as a projection oflight beam 322 on the surface. Nonetheless, the projection (“Top View”)of light beam 322 on a sample surface enables evaluating an interactionbetween light beam 322 and the sample (e.g., wafer 340). Therefore,according to embodiments of the present disclosure, controller 380 maybe configured to transform the images of the beam spot (hereinafterreferred to as “beam spot images”) taken by image sensor 360, to obtainthe projection of light beam 322 on the sample surface. The methodperformed by controller 380 is referred to as a coordinatetransformation method.

FIG. 4 is an exemplary flowchart of a process 400 for transforming abeam spot image by using the coordinate transformation method,consistent with embodiments of the present disclosure. Process 400 maybe performed by a controller, such as controller 380 illustrated in FIG.3A.

As shown in FIG. 4, at step 410, the controller may obtain a beam spotimage formed on a sample surface and captured by an image sensor (e.g.,image sensor 360 of FIG. 3A). FIG. 5A is an exemplary beams spot imagecaptured by image sensor 360 disposed at a position illustrated in FIG.3B. As shown in FIG. 5A, the shape of the beam spot at the center of theimage is distorted.

At step 420, the controller may transform coordinates of the beam spotimage to obtain a Top View of the beam spot formed on the samplesurface. The coordinate transformation may be performed based on thepositions and layouts of optical system 370, ACC module 320, and imagesensor 360. For example, controller 380 may transform the coordinates ofat least one pixel in the beam spot image based on the followingequation:

$\quad\{ \begin{matrix}{x^{\prime} = \frac{x}{- m}} \\{y^{\prime} = \frac{y}{{- m}*{\sin(\theta)}}}\end{matrix} $

where x′ and y′ are the cartesian coordinates on the wafer, x and y arethe coordinates in the beam spot image captured by image sensor 360, mis the magnification of the optical system 370, and θ is the angle ofimage sensor 360 with respect to the sample surface, as illustrated inFIG. 3A. FIG. 5B is a Top View of a beam spot formed on a samplesurface, which may be obtained by using the coordinate transformationmethod. As shown in FIG. 5B, after the coordinate transformation, theshape of the beam spot becomes less distorted.

As described previously, in some embodiments, the images are taken on arelatively rough surface, such as the surface of stage 330 illustratedin FIG. 3B. The light from light beam 322 may be scattered or diffractedinto image sensor 360 due to the rough surface. However, the scatteringrates are not uniform across the beam spot, thereby leading to adistortion of a beam profile obtained by controller 380 based on thebeam spot image captured by image sensor 360. FIG. 6 is a diagram of anexemplary 3D beam profile obtained based on a beam spot image formed ona relatively rough sample surface. In the diagram of FIG. 6, thehorizontal axes represent positions on a sample surface, and thevertical axis represents the light intensity (e.g., grey level) of thebeam spot at different positions. As shown in FIG. 6, the intensity ofthe beam spot formed on the relatively rough sample surface is notsmoothly distributed.

To minimize or eliminate the influences of the rough surface on the beamprofile, according to embodiments of the present disclosure, imagesensor 360 may be configured to capture a plurality of beam spot imagesat different locations on a sample surface, and controller 380 mayobtain an averaged image based on the plurality of images. This methodis referred to as a locations average method.

FIG. 7 is an exemplary flowchart of a process 700 for obtaining anaveraged beam spot image, consistent with embodiments of the disclosure.Process 700 may be performed by a controller, such as controller 380illustrated in FIG. 3A.

As shown in FIG. 7, at step 710, the controller may obtain a pluralityof images of a beam spot formed at different locations on a samplesurface. The locations selected for performing the locations averagemethod may be, but not limited to, a 4×4 or 5×5 matrix. FIG. 8A is anexample of a 4×4 matrix of locations 1 through 16 where beam spot imagescan be captured in the locations average method. For example, in the 4×4matrix illustrated in FIG. 8A, the controller may first move a stage(e.g., stage 330 of FIG. 3A) to location 1 such that a light beam (e.g.,light beam 322 of FIG. 3A) from an ACC module (e.g., ACC module 320 ofFIG. 3A) forms a beam spot at location 1, and an image sensor (e.g.,image sensor 360 of FIG. 3A) may capture an image of the beam spotformed at location 1. Then, the controller may move the stage tolocation 2 while keeping the positions of the ACC module and the imagesensor unchanged, such that the light beam from the ACC module forms abeam spot at location 2, and the image sensor may capture an image ofthe beam spot formed at location 2. The process continues for locations3 through 16 until the image sensor captures an image of a beam spotformed at location 16. As a result, the controller may obtain sixteen(16) beam spot images formed at locations 1 through 16, respectively.

Referring back to FIG. 7, at step 720, the controller may generate anaveraged image based on the plurality of images. In the example of the4×4 matrix illustrated in FIG. 8A, the controller may generate anaveraged image based on the sixteen (16) beam spot images taken atlocations 1 through 16. In some embodiments, the beam spot images may begreyscale images each including plurality of pixels, and each pixel hasa grey level. The controller may calculate, for at least one pixel, anaveraged grey level based on the grey levels of all corresponding pixelsin the plurality of images. For example, in the example of the 4×4matrix illustrated in FIG. 8A, for a pixel at a position (x, y), thecontroller may sum up the grey levels of all of the pixels at theposition (x, y) in all of the sixteen (16) images, and divide theobtained grey level by 16 to obtain an averaged grey level for thepixel. FIG. 8B is a diagram of an exemplary 3D beam profile obtainedbased on an averaged image that is generated based on the sixteen (16)beam spot images captured at locations 1 through 16. As shown in FIG.8B, the intensity of the beam spot in the averaged image is smoothlydistributed.

Typically, an image sensor has a dynamic range of 8 bits, 10 bits, or 16bits. Because a light beam (e.g., laser beam) may have a high intensity,or the beam intensity may vary dramatically from the center to the edgeof a light beam, it may be hard to obtain the entire beam profile (i.e.,entire intensity distribution) of the light beam by an image sensor dueto the limited dynamic range of the image sensor. According to someembodiments of the present disclosure, to overcome the limited dynamicrange of image sensors and to avoid using over-exposure pixelinformation, the complete beam profile may be restored by using imagesof a beam spot taken at different exposure times. As used herein, theexposure time represents the length of time when a film or a digitalsensor in an image sensor is exposed to light. This method may bereferred to as a dynamic range extension method.

FIG. 9 is a flowchart of a process 900 for obtaining a beam profileusing the dynamic range extension method, consistent with embodiments ofthe present disclosure. Process 900 may be performed by a controller,such as controller 380 illustrated in FIG. 3A.

As shown in FIG. 9, at step 910, the controller may first obtain asequence of images of a beam spot captured at different exposure timesof an image sensor, such as image sensor 360 of FIG. 3A. That is, eachimage of the sequence of images has been captured by the image sensor ata different exposure time of the image sensor. The exposure times mayrange from the shorted exposure time that is available for the imagesensor, to an exposure time that is long enough to capture an edge ofthe beam spot. The different exposure times may be selected to ensurethat partial beam profiles obtained from the sequence of images overlapwith each other. FIGS. 10A-10C are examples of images of a beam spotcaptured at exposure times t1, t2, and t3. FIGS. 10D-10F are examples ofpartial beam profiles obtained from the images of FIGS. 10A-10C,respectively. Exposure time t1 is the shortest time among t1, t2, andt3. Exposure time t1 may yield a first partial beam profile at a highlevel, as shown in FIG. 10D. Then, by increasing the exposure time tot2, a second partial beam profile at a middle level as shown in FIG. 10Ecan be obtained. Next, the exposure time is increased to t3, which mayyield a third partial beam profile at a low level, as shown in FIG. 10F.The partial beam profiles at FIGS. 10D-10F partially overlap with eachother.

At step 920, the controller may adjust at least one beam spot image byusing a grey level magnification factor for the at least one beam spotimage. Here, in order to overcome limited dynamic range of the imagesensor, the controller may first sort out useful pixel information byselecting pixels (also referred to as “pixels containing usefulinformation”) within or without a dynamic range from all of the pixelsin the image. The pixels without useful information (also referred to as“un-useful pixels”) may be discarded or ignored by the controller. Inother words, in some embodiments, only pixels with useful informationare used in processing. The controller may adjust the beam spot imageby, for example, multiplying a grey level of each pixel containinguseful information in the beam spot image by a grey level magnificationfactor for the image.

In some embodiments, the controller may determine a grey levelmagnification factor for a beam spot image based on an exposure time atwhich the beam spot image is captured. The grey level magnificationfactor may be inversely related to the exposure time. The longer theexposure time, the lower the grey level magnification factor is; andvice versa. For example, a grey level magnification factor for a firstselected beam spot image may be calculated by dividing a first exposuretime at which the first selected beam spot image is captured by a secondexposure time at which a second selected beam spot image is captured.

In some other embodiments, the controller may determine a grey levelmagnification factor based on the grey levels of the same pixel atdifferent exposure times. For example, the controller may determine agrey level magnification factor associated with a beam spot image bycomparing grey levels of the pixels in the beam spot image with greylevels of the pixels in other beam spot images. FIGS. 11A-11G illustratean exemplary method of determining a grey level magnification factor,according to these embodiments.

Specifically, FIG. 11A is a first exemplary beam spot image collected atexposure time t1. FIG. 11B is a second exemplary beam spot imagecollected at exposure time t2. FIG. 11C is a diagram of grey levels ofpixels (i.e., useful pixels) in the first and second beam spot images ofFIGS. 11A and 11B, when viewed along an x-axis on a sample surface wherethe beam spot is formed. In the diagram of FIG. 11C, the values on thehorizontal axis represent positions along the x-axis, and the values onthe vertical axis represent grey levels. FIG. 11D is a top view of greylevels of the useful pixels in the first and second beam spot images ofFIGS. 11A and 11B. In the diagram of FIG. 11D, the values on thehorizontal axis represent positions along the x-axis, and the values onthe vertical axis represent positions along a y-axis perpendicular tothe x-axis. Controller 380 may compare the grey levels of the first andsecond beam spot images to obtain a plurality of overlapping pixelpairs. The pixels in each overlapping pixel pair have same positions inthe corresponding beam spot images. FIG. 11E is a diagram of grey levelsof the plurality of overlapping pixel pairs, when viewed along thex-axis on the sample surface where the beam spot is formed. FIG. 11F isa top view of grey levels of the plurality of overlapping pixel pairs.The controller may determine the grey level magnification factorassociated with the first beam spot image based on the grey levels ofthe overlapping pixel pairs by using at least one of two exemplarymethods described as follows.

In the first exemplary method, the controller may determine anindividual grey level magnification factor for each overlapping pixelpair based on a ratio of the grey levels of the pair of overlappingpixels. For example, for an overlapping pixel pair consisting of a firstpixel from the first beam spot image and a second pixel from the secondbeam spot image, the controller may determine the individual grey levelmagnification factor by, for example, dividing the grey level of thesecond pixel by the grey level of the first pixel. The controller maythen determine the grey level magnification factor associated with thefirst beam spot image by, for example, averaging the individual greylevel magnification factors of all of the plurality of overlapping pixelpairs.

In the second exemplary method, assuming that the plurality ofoverlapping pixel pairs consist of a first set of pixels from the firstbeam spot image and a second set of pixels from the second beam spotimage, the controller may obtain a first total grey level by summing thegrey levels of the first set of pixels, and obtain a second total greylevel by summing the grey levels of the second set of pixels. Thecontroller may then determine the grey level magnification factorassociated with the first beam spot image by dividing the first totalgrey level by the second total grey level.

After determining the grey level magnification factor associated withthe first beam spot image, the controller may adjust the first beam spotimage by, for example, multiplying a grey level of each pixel containinguseful information in the first beam spot image by the grey levelmagnification factor. The controller may then combine the grey levels inthe adjusted first beam spot image and the grey levels in the secondbeam spot image to obtain a beam profile. FIG. 11G is a diagram of greylevels in the beam profile obtained by controller 380.

Referring back to FIG. 9, at step 930, the controller may combine thesequence of images to obtain a beam profile. For example, the controllermay combine the beam spot images in FIGS. 10A-10C by aligning thepartial beam profiles in FIGS. 10D-10F obtained from the beam spotimages and obtain the complete beam profile shown in FIG. 10G. Thecomplete beam profile can be used to monitor the beam quality.

In the example illustrated in FIGS. 10A-10G, three (3) beam spot imagesare used to obtain the complete beam profile. The present disclosure isnot limited thereto, and less or more images taken at different exposuretimes may be used to obtain the complete beam profile. Usually, 5 to 13beam spot images may be used. In some cases where the dynamic range ofan image sensor is large enough, a single beam spot image may besufficient to generate the complete beam profile.

According to some embodiments of the present disclosure, the power of alight beam (“beam power”) can also be determined based on a beam spotimage captured by an image sensor. This method is referred to as a powercalibration method. FIG. 12 is a flow chart of a process 1200 ofdetermining a beam power using the power calibration method, consistentwith such embodiments. Process 1200 may be performed by a controller,such as controller 380 in FIG. 3A.

As shown in FIG. 12, at step 1210, the controller may obtain a beam spotimage of a light beam captured by an image sensor, such as image sensor360 in FIG. 3A. At step 1220, the controller may obtain a beam profileof the light beam based on the beam spot image. For example, thecontroller may obtain the beam profile using the method described withrespect to FIG. 9. At step 1230, the controller may obtain a total greylevel of the beam spot by, for example, summing the grey levels of allpixels in the beam profile. At step 1240, the controller may determine apower of the light beam based on a relationship between the beam powerand the total grey level.

The total grey level GL can be written as:

GL=Power*β*η

where Power is the power of the beam spot from on a diffraction surface,which could be measured by a power meter, β is an energy diffractionefficiency onto the image sensor, η is a power to grey level conversionratio. The β*η represents a relationship between the Power and the totalgrey level GL. The β*η may be calibrated before operation of aninspection system (e.g., inspection system 100) and may be stored in amemory.

For example, to calibrate the β*η, the controller may first obtain animage of a sample beam spot (hereinafter referred to as a “sample beamspot image”) of a sample light beam captured by an image sensor, such asimage sensor 360 of FIG. 3A. The beam spot image may be captured insideor outside a vacuum chamber. The controller may obtain a beam profile ofthe sample light beam based on the beam spot image. FIG. 13A is anexample of a 3D beam profile that may be obtained by the controller.After the beam profile of the sample light beam is obtained, thecontroller may obtain a total grey level of the sample beam spot by, forexample, summing the grey levels of the pixels in the beam profile ofthe sample light beam. The controller may then obtain a power of asample light beam measured by a power meter. FIG. 13B schematicallyillustrates a method of measuring a beam power by a power meter,consistent with some embodiments of the present disclosure. As shown inFIG. 13B, an ACC module 1300 may be configured to emit a light beam 1310that directly enters a power meter 1320, and power meter 1320 maymeasure a beam power of light beam 1310. After the controller obtainsthe beam power of the sample light beam, the controller may determinethe β*η, which represents a relationship between the total grey level ofthe sample beam spot and the power of the sample light beam. FIG. 13C isa diagram of an exemplary relationship between the beam power and totalgrey level of a sample light beam. In the diagram of FIG. 13C, thehorizontal axis represents the beam power, and the vertical axisrepresents the total grey level of the beam spot. By doing this, thetotal grey level of the beam spot can be related with the beam power.Then the beam power of an ACC module can be monitored with the totalgrey level counts of a beam spot image captured by an image sensor insitu.

According to the above disclosed embodiments, a beam monitoring systemfor monitoring a light beam emitted from an ACC module in an inspectionsystem may include an image sensor configured to collect a sequence ofimages of a beam spot formed by the light beam at different exposuretimes of the image sensor, and a controller configured to combine thesequence of images to obtain a beam profile of the light beam. As aresult, even if the image sensor has a dynamic range that is notsufficiently large to capture the complete beam profile of the lightbeam, the controller can still obtain the complete beam profile based onthe partial beam files obtained from the sequence of images.

In addition, the controller may be configured to, for at least one ofthe sequence of images, transform coordinates of the image based onpositions of the light and the image sensor with respect to the beamspot and a magnification factor of an optical system arranged betweenthe image sensor and the surface. As a result, a projection of the lightbeam on the surface may be obtained, and an interaction between thelight beam and the surface can be accurately evaluated.

Moreover, the image sensor may be configured to, at least one exposuretime, collect a plurality of images of the beam spot formed at differentlocations on the surface, and the controller may be configured togenerate an averaged image based on the plurality of images and use theaveraged image as the collected image for the at least one exposuretime. As a result, the beam profile obtained based on the averaged imagemay be smooth, even if the surface on which the beam spot is formed isrelatively rough.

Furthermore, the controller may be configured to obtain a total greylevel of the beam spot by summing the grey levels of all pixels in thebeam profile and determine a power of the light beam based on apredetermined relationship between the total grey level and the power.As a result, the beam power may be obtained in situ while the inspectionsystem is operating, without using a power meter.

The embodiments may further be described using the following clauses:

1. A system for monitoring a beam in an inspection system, comprising:

an image sensor that includes circuitry to collect a sequence of imagesof a beam spot of a beam formed on a surface, each image of the sequenceof images having been collected at a different exposure time of theimage sensor; and

a controller having one or more processors and a memory, the controllerconfigured to combine the sequence of images to obtain a beam profile ofthe beam.

2. The system of clause 1, wherein the controller is further configuredto adjust an image of the sequence of images by using a grey levelmagnification factor associated with the image.

3. The system of clause 2, wherein the controller is further configuredto adjust the image by multiplying a grey level of each pixel in theimage that contains useful information by the grey level magnificationfactor associated with the image.

4. The system of either one of clauses 2 and 3, wherein the controlleris configured to determine the grey level magnification factorassociated with the image based on an exposure time at which the imageis collected.

5. The system of either one of clauses 2 and 3, wherein the controlleris configured to determine the grey level magnification factorassociated with the image based on a comparison of grey levels of pixelsin the sequence of images.

6. The system of clause 5, wherein the image is a first selected imageof the sequence of images, and the controller is configured to determinethe grey level magnification factor associated with the first selectedimage based on:

a comparison of the first selected image with a second selected image ofthe plurality of images to obtain a plurality of overlapping pixelpairs; and

a determination of the grey level magnification factor associated withthe first selected image based on grey levels of pixels in the pluralityof overlapping pixel pairs.

7. The system of any one of clauses 1 to 6, wherein the controller isfurther configured to, for a particular image of the sequence of images,transform coordinates of the particular image based on positions of thebeam and the image sensor with respect to the beam spot and amagnification factor of an optical system arranged between the imagesensor and the surface.

8. The system of any one of clauses 1 to 7, wherein

the image sensor includes circuitry to, at an exposure time, collect aplurality of images of the beam spot formed at different locations onthe surface, and

the controller is configured to:

-   -   generate an averaged image based on the plurality of images; and    -   use the averaged image as the collected image for the exposure        time.

9. The system of clause 8, wherein the controller is configured togenerate the averaged image by:

for a pixel, determine an averaged grey level based on the grey levelsof pixels in the plurality of images.

10. The system of any one of clauses 1 to 9, wherein the controller isfurther configured to:

obtain a total grey level of the beam spot by summing the grey levels ofall pixels in the beam profile; and

determine a power of the beam based on a relationship between the totalgrey level and the power.

11. The system of clause 10, wherein the controller is configured todetermine the relationship between the total grey level and the powerby:

obtaining a power of a sample beam measured by a power meter;

obtaining an image of a sample beam spot formed by the sample beam;

obtaining a beam profile of the sample beam based on the image of thesample beam;

obtaining a total grey level of the sample beam spot by summing the greylevels of the pixels in the beam profile of the sample beam;

determining a relationship between the total grey level of the samplebeam spot and the power of the sample beam; and

determining the relationship between the total grey level and the powerbased on the relationship between the total grey level of the samplebeam spot and the power of the sample beam.

12. The system of either one of clauses 10 and 11, wherein

the inspection system further includes an emitter that includescircuitry to emit the beam, and

the controller is further configured to adjust an output power of theemitter based on the determined power of the beam.

13. The system of any one of clauses 1 to 12, wherein

the inspection system further includes an emitter that includescircuitry to emit the beam, and the emitter includes a beam shaper, and

the controller is further configured to control the beam shaper based onthe beam profile of the beam.

14. The system of any one of clauses 1 to 13, wherein the inspectionsystem further includes a charged particle beam tool that includescircuitry to emit a charged particle beam onto the surface.

15. The system of clause 14, wherein the controller is furtherconfigured to align the beam spot of the beam to a field of view of thecharged particle beam tool.

16. The system of clause 14, wherein the inspection system furtherincludes an Advanced Charge Control module that includes circuitry toemit the beam.

17. The system of clause 16, wherein the charged particle beam tool, theAdvanced Charge Control module, and the image detector are disposedoutside of a vacuum chamber.

18. A method of monitoring a beam in an inspection system, comprising:

obtaining a sequence of images of a beam spot of a beam formed on asurface, each image of the sequence of images having been collected byan image sensor at a different exposure time of the image sensor; and

combining the sequence of images to obtain a beam profile of the beam.

19. The method of clause 18, further comprising adjusting an image ofthe sequence of images by using a grey level magnification factorassociated with the image.

20. The method of clause 19, wherein adjusting the image of the sequenceof images comprises:

multiplying a grey level of each pixel in the image that contains usefulinformation by the grey level magnification factor associated with theimage.

21. The method of either one of clauses 19 and 20, further comprisingdetermining the grey level magnification factor associated with theimage based on an exposure time at which the image is collected.

22. The method of either one of clauses 19 and 20, further comprisingdetermining the grey level magnification factor associated with theimage based on a comparison of grey levels of pixels in the sequence ofimages.

23. The method of clause 22, wherein the image is a first selected imageof the sequence of images, and the method further comprises:

comparing the first selected image with a second selected image of theplurality of images to obtain a plurality of overlapping pixel pairs;and

determining the grey level magnification factor associated with thefirst selected image based on grey levels of pixels in the plurality ofoverlapping pixel pairs.

24. The method of any one of clauses 18 to 23, further comprising, for aparticular image of the sequence of images, transforming coordinates ofthe particular image based on positions of the beam and the image sensorwith respect to the beam spot and a magnification factor of an opticalsystem arranged between the image sensor and the surface.

25. The method of any one of clauses 18 to 24, further comprising:

obtaining a plurality of images of the beam spot formed at differentlocations on the surface; and

generating an averaged image based on the plurality of images.

26. The method of clause 25, wherein the generating the averaged imagecomprising:

for a pixel, determining an averaged grey level based on the grey levelsof the pixels in the plurality of images.

27. The method of any one of clauses 18 to 26, further comprising:

obtaining a total grey level of the beam spot by summing the grey levelsof all of the pixels in the beam profile; and

determining a power of the beam based on a predetermined relationshipbetween the total grey level and the power.

28. The method of clause 27, further comprising determining therelationship between the total grey level and the power by:

obtaining a power of a sample beam measured by a power meter;

obtaining an image of a sample beam spot formed by the sample beam;

obtaining a beam profile of the sample beam based on the image of thesample beam;

obtaining a total grey level of the sample beam spot by summing the greylevels of the pixels in the beam profile of the sample beam;

determining a relationship between the total grey level of the samplebeam spot and the power of the sample beam; and

determining the relationship between the total grey level and the powerbased on the relationship between the total grey level of the samplebeam spot and the power of the sample beam.

29. The method of either one of clauses 27 and 28, wherein

the inspection system further includes an emitter that emits the beam,and

the method further comprises adjusting an output power of the emitterbased on the determined power of the beam.

30. The method of any one of clauses 18 to 29, wherein

the inspection system further includes an emitter that emits the beam,and the emitter includes a beam shaper, and

the method further comprises controlling the beam shaper based on thebeam profile of the beam.

31. The method of any one of clauses 18 to 30, wherein

the inspection system further includes an emitter that emits the beamand a charged particle beam tool that emits a charged particle beam ontothe surface,

the method further comprises aligning the beam spot of the beam to afield of view of the charged particle beam tool.

32. A non-transitory computer-readable medium that stores a set ofinstructions that is executable by a processor of a system formonitoring a beam in an inspection system to cause the system to performa method, the method comprising:

obtaining a sequence of images of a beam spot of a beam formed on asurface, each image of the sequence of images having been collected byan image sensor at a different exposure time of the image sensor; and

combining the sequence of images to obtain a beam profile of the beam.

33. The medium of clause 32, wherein the set of instructions that isexecutable by the processor of the system further causes the system toperform:

adjusting an image of the sequence of images by using a grey levelmagnification factor associated with the image.

34. The medium of clause 33, wherein adjusting the image of the sequenceof images comprises:

multiplying a grey level of each pixel in the image that contains usefulinformation by the grey level magnification factor associated with theimage.

35. The medium of either one of clauses 33 and 34, wherein the set ofinstructions that is executable by the processor of the system furthercauses the system to perform:

determining the grey level magnification factor associated with theimage based on an exposure time at which the image is collected.

36. The medium of either one of clauses 33 and 34, wherein the set ofinstructions that is executable by the processor of the system furthercauses the system to perform:

determining the grey level magnification factor associated with theimage based on a comparison of grey levels of pixels in the sequence ofimages.

37. The medium of clause 36, wherein the image is a first selected imageof the sequence of images, and the set of instructions that isexecutable by the processor of the system further causes the system toperform:

comparing the first selected image with a second selected image of theplurality of images to obtain a plurality of overlapping pixel pairs;and

determining the grey level magnification factor associated with thefirst selected image based on grey levels of pixels in the plurality ofoverlapping pixel pairs.

38. The medium of any one of clauses 18 to 23, wherein the set ofinstructions that is executable by the processor of the system furthercauses the system to perform:

for a particular image of the sequence of images, transformingcoordinates of the particular image based on positions of the beam andthe image sensor with respect to the beam spot and a magnificationfactor of an optical system arranged between the image sensor and thesurface.

39. The medium of any one of clauses 32 to 38, wherein the set ofinstructions that is executable by the processor of the system furthercauses the system to perform:

obtaining a plurality of images of the beam spot formed at differentlocations on the surface; and

generating an averaged image based on the plurality of images.

40. The medium of clause 39, wherein generating the averaged imagecomprising:

for a pixel, determining an averaged grey level based on the grey levelsof the pixels in the plurality of images.

41. The medium of any one of clauses 32 to 40, wherein the set ofinstructions that is executable by the processor of the system furthercauses the system to perform:

obtaining a total grey level of the beam spot by summing the grey levelsof all of the pixels in the beam profile; and

determining a power of the beam based on a predetermined relationshipbetween the total grey level and the power.

42. The medium of clause 41, wherein the set of instructions that isexecutable by the processor of the system further causes the system toperform:

determining the relationship between the total grey level and the powerby:

-   -   obtaining a power of a sample beam measured by a power meter;    -   obtaining an image of a sample beam spot formed by the sample        beam;    -   obtaining a beam profile of the sample beam based on the image        of the sample beam;    -   obtaining a total grey level of the sample beam spot by summing        the grey levels of the pixels in the beam profile of the sample        beam;    -   determining a relationship between the total grey level of the        sample beam spot and the power of the sample beam; and    -   determining the relationship between the total grey level and        the power based on the relationship between the total grey level        of the sample beam spot and the power of the sample beam.

43. The medium of either one of clauses 41 and 42, wherein theinspection system further includes an emitter that emits the beam, andthe set of instructions that is executable by the processor of thesystem further causes the system to perform:

adjusting an output power of the emitter based on the determined powerof the beam.

44. The medium of any one of clauses 32 to 43, wherein the inspectionsystem further includes an emitter that emits the beam, and the emitterincludes a beam shaper, and the set of instructions that is executableby the processor of the system further causes the system to perform:

controlling the beam shaper based on the beam profile of the beam.

45. The medium of any one of clauses 18 to 30, wherein the inspectionsystem further includes an emitter that emits the beam and a chargedparticle beam tool that emits a charged particle beam onto the surface,the set of instructions that is executable by the processor of thesystem further causes the system to perform:

aligning the beam spot of the beam to a field of view of the chargedparticle beam tool.

46. A system for monitoring a beam in an inspection system, comprising:

an image sensor that includes circuitry to collect an image of a beamspot of a beam formed on a surface; and

a controller having one or more processors and a memory, the controllerconfigured to transform coordinates of the image based on positions ofthe beam and the image sensor with respect to the beam spot and amagnification factor of an optical system arranged between the imagesensor and the surface.

47. A method of monitoring a beam in an inspection system, comprising:

collecting, by an image sensor, an image of a beam spot of a beam formedon a surface; and

transforming coordinates of the image based on positions of the beam andthe image sensor with respect to the beam spot and a magnificationfactor of an optical system arranged between the image sensor and thesurface.

48. A non-transitory computer-readable medium that stores a set ofinstructions that is executable by a processor of a system formonitoring a beam in an inspection system to cause the system to performa method, the method comprising:

receiving an image of a beam spot of a beam formed on a surface andcollected by an image sensor; and

transforming coordinates of the image based on positions of the beam andthe image sensor with respect to the beam spot and a magnificationfactor of an optical system arranged between the image sensor and thesurface

49. A system for monitoring a beam in an inspection system, comprising:

an image sensor that includes circuitry to collect a plurality of imagesof a beam spot of a beam, each image of the plurality of images beingformed at a different location on a surface; and

a controller having one or more processors and a memory, the controllerconfigured to generate an averaged image based on the plurality ofimages.

50. The system of clause 49, wherein the controller is configured togenerate the averaged image by:

for a pixel, determine an averaged grey level based on the grey levelsof the pixels in the plurality of images.

51. A method of monitoring a beam in an inspection system, comprising:

collecting a plurality of images of a beam spot of a beam, each image ofthe plurality of images being formed at a different location on asurface; and

generating an averaged image based on the plurality of images.

52. The method of clause 51, wherein the generating the averaged imagecomprises:

for a pixel, determining an averaged grey level based on the grey levelsof the pixels in the plurality of images.

53. A non-transitory computer-readable medium that stores a set ofinstructions that is executable by a processor of a system formonitoring a beam in an inspection system to cause the system to performa method, the method comprising:

receiving a plurality of images of a beam spot of a beam, each image ofthe plurality of images being formed at a different location on asurface; and

generating an averaged image based on the plurality of images.

54. The medium of clause 53, wherein the generating the averaged imagecomprises:

for a pixel, determining an averaged grey level based on the grey levelsof the pixels in the plurality of images.

55. A system for monitoring a beam in an inspection system, comprising:

an image sensor that includes circuitry to collect an image of a beamspot of a beam formed on a surface; and

a controller having one or more processors and a memory, the controllerconfigured to:

-   -   obtain a beam profile of the beam based on the image;    -   obtain a total grey level of the beam spot based on the beam        profile; and    -   determine a power of the beam based on a predetermined        relationship between the total grey level and the power.

56. The system of clause 55, wherein the controller is configured todetermine the predetermined relationship between the total grey leveland the power by:

obtaining a power of a sample beam measured by a power meter;

obtaining an image of a sample beam spot formed by the sample beam;

obtaining a beam profile of the sample beam based on the image of thesample beam;

obtaining a total grey level of the sample beam spot by summing the greylevels of the pixels in the beam profile of the sample beam;

determining a relationship between the total grey level of the samplebeam spot and the power of the sample beam; and

determining the relationship between the total grey level and the powerbased on the relationship between the total grey level of the samplebeam spot and the power of the sample beam.

57. The system of either one of clauses 55 and 56, wherein

the inspection system further includes an emitter that includescircuitry to emit the beam, and

the controller is further configured to adjust an output power of theemitter based on the determined power of the beam.

58. The system of any one of clauses 55 to 57, wherein the inspectionsystem further includes a charged particle beam tool that that includescircuitry to emit a charged particle beam onto the surface.

59. The system of clause 58, wherein the inspection system furtherincludes an Advanced Charge Control module that includes circuitry toemit the beam.

60. The system of clause 59, wherein the charged particle beam tool, theAdvanced Charge Control module, and the image detector are disposedoutside of a vacuum chamber.

61. A method of monitoring a beam in an inspection system, comprising:

collecting an image of a beam spot of a beam formed on a surface;

obtaining a beam profile of the beam based on the image;

obtaining a total grey level of the beam spot based on the beam profile;and

determining a power of the beam based on a predetermined relationshipbetween the total grey level and the power.

62. The method of clause 61, further comprising determining therelationship between the total grey level and the power by:

obtaining a power of a sample beam measured by a power meter;

obtaining an image of a sample beam spot formed by the sample beam;

obtaining a beam profile of the sample beam based on the image of thesample beam;

obtaining a total grey level of the sample beam spot by summing the greylevels of the pixels in the beam profile of the sample beam;

determining a relationship between the total grey level of the samplebeam spot and the power of the sample beam; and

determining the relationship between the total grey level and the powerbased on the relationship between the total grey level of the samplebeam spot and the power of the sample beam.

63. A non-transitory computer-readable medium that stores a set ofinstructions that is executable by a processor of a system formonitoring a beam in an inspection system to cause the system to performa method, the method comprising:

receiving an image of a beam spot of a beam formed on a surface;

obtaining a beam profile of the beam based on the image;

obtaining a total grey level of the beam spot based on the beam profile;and

determining a power of the beam based on a predetermined relationshipbetween the total grey level and the power.

64. The non-transitory computer-readable medium of clause 63, whereinthe set of instructions that is executable by the processor of thesystem further causes the system to perform:

determining the relationship between the total grey level and the powerby:

-   -   receiving a power of a sample beam measured by a power meter;    -   receiving an image of a sample beam spot formed by the sample        beam;    -   obtain a beam profile of the sample beam based on the image of        the sample beam;    -   obtaining a total grey level of the sample beam spot by summing        the grey levels of the pixels in the beam profile of the sample        beam;    -   determining a relationship between the total grey level of the        sample beam spot and the power of the sample beam; and    -   determining the relationship between the total grey level and        the power based on the relationship between the total grey level        of the sample beam spot and the power of the sample beam.

It will be appreciated that the present invention is not limited to theexact construction that has been described above and illustrated in theaccompanying drawings, and that various modifications and changes can bemade without departing from the scope thereof. It is intended that thescope of the invention should only be limited by the appended claims.

1. A system for monitoring a beam in an inspection system, comprising:an image sensor that includes circuitry to collect a sequence of imagesof a beam spot of a beam formed on a surface, each image of the sequenceof images having been collected at a different exposure time of theimage sensor; and a controller having one or more processors and amemory, the controller configured to combine the sequence of images toobtain a beam profile of the beam.
 2. The system of claim 1, wherein thecontroller is further configured to adjust an image of the sequence ofimages by using a grey level magnification factor associated with theimage.
 3. The system of claim 2, wherein the controller is furtherconfigured to adjust the image by multiplying a grey level of each pixelin the image that contains useful information by the grey levelmagnification factor associated with the image.
 4. The system of claim2, wherein the controller is configured to determine the grey levelmagnification factor associated with the image based on an exposure timeat which the image is collected.
 5. The system of claim 2, wherein thecontroller is configured to determine the grey level magnificationfactor associated with the image based on a comparison of grey levels ofpixels in the sequence of images.
 6. The system of claim 5, wherein theimage is a first selected image of the sequence of images, and thecontroller is configured to determine the grey level magnificationfactor associated with the first selected image based on: a comparisonof the first selected image with a second selected image of theplurality of images to obtain a plurality of overlapping pixel pairs;and a determination of the grey level magnification factor associatedwith the first selected image based on grey levels of pixels in theplurality of overlapping pixel pairs.
 7. The system of claim 1, whereinthe controller is further configured to, for a particular image of thesequence of images, transform coordinates of the particular image basedon positions of the beam and the image sensor with respect to the beamspot and a magnification factor of an optical system arranged betweenthe image sensor and the surface.
 8. The system of claim 1, wherein theimage sensor includes circuitry to, at an exposure time, collect aplurality of images of the beam spot formed at different locations onthe surface, and the controller is configured to: generate an averagedimage based on the plurality of images; and use the averaged image asthe collected image for the exposure time.
 9. The system of claim 8,wherein the controller is configured to generate the averaged image by:for a pixel, determine an averaged grey level based on the grey levelsof pixels in the plurality of images.
 10. The system of claim 1, whereinthe controller is further configured to: obtain a total grey level ofthe beam spot by summing the grey levels of all pixels in the beamprofile; and determine a power of the beam based on a relationshipbetween the total grey level and the power.
 11. The system of claim 10,wherein the controller is configured to determine the relationshipbetween the total grey level and the power by: obtaining a power of asample beam measured by a power meter; obtaining an image of a samplebeam spot formed by the sample beam; obtaining a beam profile of thesample beam based on the image of the sample beam; obtaining a totalgrey level of the sample beam spot by summing the grey levels of thepixels in the beam profile of the sample beam; determining arelationship between the total grey level of the sample beam spot andthe power of the sample beam; and determining the relationship betweenthe total grey level and the power based on the relationship between thetotal grey level of the sample beam spot and the power of the samplebeam.
 12. The system of claim 10, wherein the inspection system furtherincludes an emitter that includes circuitry to emit the beam, and thecontroller is further configured to adjust an output power of theemitter based on the determined power of the beam.
 13. The system ofclaim 1, wherein the inspection system further includes an emitter thatincludes circuitry to emit the beam, and the emitter includes a beamshaper, and the controller is further configured to control the beamshaper based on the beam profile of the beam.
 14. The system of claim 1,wherein the inspection system further includes a charged particle beamtool that includes circuitry to emit a charged particle beam onto thesurface.
 15. A non-transitory computer-readable medium that stores a setof instructions that is executable by a processor of a system formonitoring a beam in an inspection system to cause the system to performa method, the method comprising: obtaining a sequence of images of abeam spot of a beam formed on a surface, each image of the sequence ofimages having been collected by an image sensor at a different exposuretime of the image sensor; and combining the sequence of images to obtaina beam profile of the beam.